234 ELECTRO-PHYSIOLOGY CHAP. 



cut end of the nerve drops upon the pericardium during diastole 

 (cf. Kuhne, I.e. p. 85). 



A twitch is often produced, as in muscle, by merely bringing 

 the cross-section of the nerve into contact with a drop of con- 

 ducting fluid. Eckhardt (12) employed this last method to 

 investigate chemical stimulation of nerve, as Kiihne had done 

 for muscle. For both we must distinguish between the electrical 

 excitation caused by short-circuiting of the demarcation current, 

 and the resulting chemical action a difficult if not impossible 

 task in many cases. The electrical origin of the twitch which 

 (as Hering noted) makes its appearance on touching a fresh cross- 

 section with a drop of 0'6 per cent salt solution, or of the con- 

 centrated solution of zinc and copper sulphate, said by Eckhardt 

 to be chemically inactive, can hardly be disputed. With the 

 very active solutions of fixed alkalies, on the other hand, where 

 we must take into consideration that the magnitude of the twitch 

 may be due solely to the fact " that they moisten the nerve more 

 rapidly and efficaciously than other fluids, and thus produce a 

 quicker electrical variation in the nerve," the same cannot be 

 asserted with equal certainty. In all experiments on excitation 

 of nerve and muscle by their intrinsic currents, great excitability 

 of the preparation is, as has been stated, indispensable ; hence 

 they come off best in the cold season. In experimenting on 

 frogs' nerves that have been kept in a cold room (at about C.), 

 Hering has again pointed out the extraordinary tendency to 

 tetanic excitation exhibited under these circumstances particu- 

 larly by H. esculenta, in a less degree by E. temporaria. The 

 simple division or constriction of the sciatic is, as a rule, sufficient 

 to induce a protracted, unbroken spasm of the leg, which is on an 

 average the more marked as the nerve is divided higher up, and 

 may be called out again after it has expired, by making a fresh 

 cross-section. 



Since these very excitable preparations fall, under the action 

 of the weakest battery currents, into a "closure tetanus" that 

 lasts during the entire passage of the current, it is not surprising 

 that, under such conditions, the mere short-circuiting of the 

 demarcation current will cause tetanic excitation as was 

 frequently observed by Hering. Vigorous closure twitches, with 

 or without subsequent clonic disturbance, occur not merely on 

 bringing a fresh cross-section of the sciatic nerve into contact 



